Magnesium alloys

ABSTRACT

A magnesium base alloy for high pressure die casting (HPDC), providing good creep and corrosion resistance, comprises: at least 91 weight percent magnesium; 0.1 to 2 weight percent of zinc; 2.1 to 5 percent of a rare earth metal component; 0 to 1 weight percent calcium; 0 to 0.1 weight percent of an oxidation inhibiting element other than calcium (e.g., Be); 0 to 0.4 weight percent zirconium, hafnium and/or titanium; 0 to 0.5 weight percent manganese; no more than 0.001 weight percent strontium; no more than 0.05 weight percent silver and no more than 0.1 weight percent aluminum; any remainder being incidental impurities. For making prototypes, gravity (e.g. sand) cast and HPDC components from the alloy have similar mechanical properties, in particular tensile strength. The temperature dependence of the latter, although negative, is much less so than for some other known alloys.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is filed under 35 U.S.C. § 371 as a national stageapplication of PCT/GB96/00261 filed Feb. 6, 1996 which claims thebenefit of British Pat. App. No. 9502238.0 filed Feb. 6, 1995.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

2. Description of the Related Art

This invention relates to magnesium alloys.

High pressure die cast (HPDC) components in magnesium base alloys havebeen successfully produced for almost 60 years, using both hot and coldchamber machines.

Compared to gravity or sand casting, HPDC is a rapid process suitablefor large scale manufacture. The rapidity with which the alloysolidifies in HPDC means that the cast product has different propertiesrelative to the same alloy when gravity cast. In particular, the grainsize is normally finer, and this would generally be expected to giverise to an increase in tensile strength with a concomitant decrease increep resistance.

Any tendency to porosity in the cast product may be alleviated by theuse of a “pore free” process (PFHPDC) in which oxygen is injected intothe chamber and is gettered by the casting alloy.

The relatively coarse grain size from gravity casting can be reduced bythe addition of a grain refining component, for example zirconium innon-aluminium containing alloys, or carbon or carbide in aluminiumcontaining alloys. By contrast, HPDC alloys generally do not need, anddo not contain, such component.

Until the mid 1960's it would be fair to say that the only magnesiumalloys used commercially for HPDC were based on the Mg—Al—Zn—Mn system,such as the alloys known as AZ91 and variants thereof. However, sincethe mid 1960's increasing interest has been shown in the use ofmagnesium base alloys for non-aerospace applications, particularly bythe automotive industry, and high purity versions of known alloys, suchas AZ91 and AM60, are beginning to be used in this market because oftheir greatly enhanced corrosion resistance.

However, both of these alloys have limited capability at elevatedtemperatures, and are unsuitable for applications operating much above100° C.

Some of the properties considered to be desirable in an HPDC alloy are:

a) Creep strength of the product at 175° C. as good as AZ91 type alloysat 150° C.

b) Room temperature strength of the product similar to AZ91 type alloys.

c) Good vibration damping.

d) Castability of the alloy similar to, or better than AZ91 type alloys.

e) Corrosion resistance of the product similar to AZ91 type alloys.

f) Thermal conductivity of the product preferably better than AZ91 typealloys.

g) Cost equivalent to AZ91 type alloys

One successful alloy development at this stage was within theMg—Al—Si—Mn system, giving alloys such as those known as AS41, AS21 andAS11; only the first of these has been fully exploited; the other two,although offering even higher creep strengths, are generally regarded asdifficult to cast, particularly since high melt temperatures arerequired. AS41 meets most of the objectives listed above, although itsliquidus temperature is about 30° C. higher than that of AZ91 typealloys.

Another series of alloys developed at about the same time included arare earth component, a typical example being AE42, comprising of theorder of 4% aluminium, 2% rare earth(s), about 0.25% manganese, and thebalance magnesium with minor components/impurities. This alloy has ayield strength which is similar at room temperature to that of AS41, butwhich is superior at temperatures greater than about 150° C. (even so,the yield strength still shows a relatively marked decrease in valuewith rising temperature, as will be mentioned again below). Moreimportantly, the creep strength of AE42 exceeds even AS21 alloy at alltemperatures up to at least 200° C.

The present invention relates to magnesium based alloys of the Mg—RE—Znsystem (RE=rare earth). Such systems are known. Thus British PatentSpecification No. 1 378 281 discloses magnesium based light structuralalloys which comprise neodymium, zinc, zirconium and, optionally, copperand manganese. A further necessary component in these alloys is 0.8 to 6weight percent yttrium. Similarly SU-443096 requires the presence of atleast 0.5% yttrium.

British Patent Specification No. 1 023 128 also discloses magnesium basealloys which comprise a rare earth metal and zinc. In these alloys, thezinc to rare earth metal ratio is from ⅓ to 1 where there is less than0.6 weight percent of rare earth, and in alloys containing 0.6 to 2weight percent rare earth metal, 0.2 to 0.5 weight percent of zinc ispresent.

More particularly British Patent Specification Nos 607588 and 637040relate to systems containing up to 5% and 10% of zinc respectively. InGB 607588, it is stated that “The creep resistance . . . is notadversely affected by the presence of zinc in small or moderate amounts,not exceeding 5 per cent for example . . . ”, and “The presence of zincin amounts of up to 5 per cent has a beneficial effect on the foundryproperties for these types of casting where it is desirable to avoidlocal4sed contraction on solidification and some dispersed unsoundnesswould be less objectionable”. A typical known system is the alloy ZE53,containing a nominal 5 percent zinc and a nominal 3 percent rare earthcomponent.

In these systems it is recognised that the rare earth component givesrise to a precipitate at grain boundaries, and enhances castability andcreep resistance, although there may be a slight decrease in tensilestrength compared to a similar alloy lacking such component. The highmelting point of the precipitate assists in maintaining the propertiesof the casting at high temperatures.

The two British patents last mentioned above refer to sand casting, andspecifically mention the desirability of the presence of zirconium inthe casting alloy as a grain refining element. To be effective for suchpurpose, the necessary amount of zirconium is said to be between 0.1 and0.9 weight percent (saturation level) (GB 607588) or between 0.4 and 0.9weight percent (GB 637040).

BRIEF SUMMARY OF THE INVENTION

As used hereinafter, by the term “rare earth” is intended any element ormixture of elements with atomic numbers 57 to 71 (lanthanum tolutetium). While lanthanum is, strictly speaking not a rare earthelement, it may or may not be present; however, “rare earth” is notintended to include elements such as yttrium.

The present invention provides a magnesium base alloy for high pressuredie casting comprising

at least 91.9 weight percent magnesium;

0.1 to 2 weight percent of zinc;

2.1 to 5 weight percent of a rare earth metal component other thanyttrium;

0 to 1 weight percent calcium;

0 to 0.1 weight percent of an oxidation inhibiting element other thancalcium;

no more than 0.001 weight percent strontium;

no more than 0.05 weight percent silver;

less than 0.1 weight percent aluminium, and

substantially no undissolved iron; any balance being incidentalimpurities.

The invention also provides a magnesium base alloy for high pressure diecasting comprising

at least 91 weight percent magnesium;

0.1 to 2 weight percent of zinc;

2.1 to 5 weight percent of a rare earth metal component other thanyttrium;

0 to 1 weight percent calcium;

0 to 0.1 weight percent of an oxidation inhibiting element other thancalcium;

0 to 0.4 weight percent zirconium, hafnium and/or titanium;

0 to 0.5 weight percent manganese;

no more than 0.001 weight percent strontium;

no more than 0.05 weight percent silver; and

no more than 0.1 weight percent aluminium.

any balance being incidental impurities.

Oxidation inhibiting elements other than calcium (e.g. beryllium),manganese, and zirconium/hafnium/titanium are optional components andtheir contribution to the composition will be discussed later.

A preferred range for zinc is 0.1 to 1 weight percent, and morepreferably 0.2 to 0.6 weight percent.

Following the ASTM nomenclature system, an alloy containing a nominal Xweight percent rare earth and Y weight percent zinc, where X and Y arerounded down to the nearest integer, and where X is greater than Y,would be referred to as an EZXY alloy.

This nomenclature will be used for prior art alloys, but alloysaccording to the invention as defined above will henceforth be termedMEZ alloys whatever their precise composition.

Compared with ZE53, MEZ alloys can exhibit improved creep and corrosionresistance (given the same thermal treatment), while retaining goodcasting properties; zinc is present in a relatively small amount,particularly in the preferred alloys, and the zinc to rare earth ratiois no greater than unity (and is significantly less than unity in thepreferred alloys) compared with the 5:3 ratio for ZE53.

Furthermore, contrary to normal expectations, it has been found that MEZalloys exhibit no very marked change in tensile strength on passing fromsand or gravity casting to HPDC. In addition the grain structure altersonly to a relatively minor extent. Thus MEZ alloys have the advantagethat there is a reasonable expectation that the properties of prototypesof articles formed by sand or gravity casting will not be greatlydifferent from those of such articles subsequently mass produced byHPDC.

By comparison, HPDC AE42 alloys show a much finer grain structure, andan approximately threefold increase in tensile strength at roomtemperature, to become about 40% greater than MEZ alloys. However, thetemperature dependence of tensile strength, although negative for bothtypes of alloy, is markedly greater for AE42 alloys than for MEZ alloys,with the result that at above about 150° C. the MEZ alloys tend to havegreater tensile strength.

Furthermore, the creep strength of HPDC AE42 alloys is markedly lowerthan that of HPDC MEZ alloys at all temperatures up to at least 177° C.

Preferably the balance of the alloy composition, if any, is less than0.15 weight percent.

The rare earth component could be cerium, cerium mischmetal or ceriumdepleted mischmetal. A preferred lower limit to the range is 2.1 weightpercent. A preferred upper limit is 3 weight percent.

An MEZ alloy preferably contains minimal amounts of iron, copper andnickel, to maintain a low corrosion rate. There is preferably less than0.005 weight percent of iron. Low iron can be achieved by addingzirconium, (for example in the form of Zirmax, which is a 1:2 alloy ofzirconium and magnesium) effectively to precipitate the iron from themolten alloy; once cast, an MEZ alloy can comprise a residual amount ofup to 0.4 weight percent zirconium, but preferred and most preferredupper limits for this element are 0.2 and 0.1 weight percentrespectively. Preferably a residue of at least 0.01 weight percent ispresent. Zirmax is a registered trademark of Magnesium Elektron Limited.

Particularly where at least some residual zirconium is present, thepresence of up to 0.5 weight percent manganese may also be conducive tolow iron and reduces corrosion. Thus, as described in greater detailhereinafter, the addition of as much as about 0.8 weight percent ofzirconium (but more commonly 0.5 weight per cent) might be required toachieve an iron content of less than 0.003 weight percent; however, thesame result can be achieved with about 0.06 weight percent of zirconiumif manganese is also present. An alternative agent for removing iron istitanium.

The presence of calcium is optional, but is believed to give improvedcasting properties. A minor amount of an element such as beryllium maybe present, preferably no less than 0.0005 weight percent, andpreferably no more than 0.005 weight percent, and often around 0.001weight percent, to prevent oxidation of the melt. However, if it isfound that such element (for example beryllium) is removed by the agent(for example zirconium) which is added to remove the iron, substitutionthereof by calcium might in any case be necessary. Thus calcium can actas both anti-oxidant and to improve casting properties, if necessary.

Preferably there is less than 0.05 weight per cent, and more preferablysubstantially no aluminium in the alloy. Preferably the alloy containsno more than 0.1 weight percent of each of nickel and copper, andpreferably no more than 0.05 weight percent copper and 0.005 weightpercent nickel. Preferably there is substantially no strontium in thealloy. Preferably the alloy comprises substantially no silver.

As cast, MEZ alloys exhibit a low corrosion rate, for example of lessthan 2.50 mm/year (100 mils/year) (ASTM B117 Salt Fog Test). Aftertreatment T5 (24 hours at 250° C.) the corrosion rate is still low.

As cast, an MEZ alloy may have a creep resistance such that the time toreach 0.1 percent creep strain under an applied stress of 46 MPa at 177°C. is greater than 500 hours; after treatment T5 the time may still begreater than 100 hours.

The invention will be further illustrated by reference to theaccompanying Figures, and by reference to the appended Tables which willbe described as they are encountered. In the Figures:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows the grain structure of gravity cast ZE53 with highzirconium, melt DF2218;

FIG. 2 shows the grain structure of gravity cast ZE53 with manganeseadded, melt DF2222;

FIG. 3 shows the grain structure of gravity cast MEZ with highzirconium, melt DF2220;

FIG. 4 shows the grain structure of gravity cast MEZ with manganeseadded, melt DF2224; and

FIG. 5 shows the grain structure of gravity cast MEZ with low zirconium,melt DF2291.

FIG. 6 illustrates and compares the tensile properties of pore free HPDCalloys MEZ and AE42;

FIG. 7 illustrates and compares the tensile properties of HPDC MEZ andpore free HPDC (PFHPDC) alloys MEZ;

FIG. 8 illustrates the effect of heat treatment on the tensileproperties of PFHPDC MEZ at various temperatures;

FIG. 9 shows the results of measuring creep resistance of PFHPDC MEZ,AE42 and ZC71 under various conditions of stress and temperature;

FIG. 10 shows the grain structure of PFHPDC MEZ in the as cast (F)condition;

FIG. 11 shows the grain structure of PFHPDC MEZ in the T6 heat treatedcondition; and

FIG. 12 shows the porosity of MPDC MEZ.

DETAILED DESCRIPTION OF THE DRAWING

The condition F is “as cast”, and T5 treatment involves maintaining thecasting at 250° C. for 24 hours. For T6 treatment the casting is held at420° C. for 2 hours, quenched into hot water, held at 180° C. for 18hours and cooled in air.

An initial investigation was made into the properties of MEZ alloys andZE53 alloys in the gravity cast state.

Table 1 relates to ZE53 and MEZ alloys, and indicates the effect ofmanganese or zirconium addition on the iron, manganese and zirconiumcontent of the resulting alloy.

The first eight of the compositions of Table 1 comprise four variationsof each of the alloys MEZ and ZES3. One set of four compositions hasmanganese added to control the iron content, and the other set has arelatively high zirconium addition (saturation is about 0.9 weightpercent) for the same purpose, and arrow bars were gravity casttherefrom. A different set of four selected from these eightcompositions is in the as cast state, with the complementary set in theT5 condition.

Table 2 indicates the compositions and states of these eight alloys inmore detail, and measurements of the tensile strength of the arrow bars.

Table 3 gives comparative data on creep properties of these eight alloysMEZ and ZE53 in the form of the gravity cast arrow bars.

Table 4 gives comparative data on corrosion properties of the eightalloy compositions in the form of the gravity cast arrow bars, andillustrates the effect of T5 treatment on the corrosion rate.

Corrosion data on another two of the alloys listed in Table 1 iscontained in Table 5, measurements being taken on a sequence of arrowbars from each respective single casting. In addition to the elementsshown in the Table, each of alloys 2290 and 2291 included 2.5 weightpercent rare earth, and 0.5 weight percent zinc. This table is worthy ofcomment, since it shows that those bars which are first cast are moreresistant to corrosion than those which are cast towards the end of theprocess. While not wishing to be bound to any theory, it seems possiblethat the iron is precipitated by the zirconium, and that the precipitatetends to settle from the liquid phase, so that early bars are depletedin iron relative to later castings.

FIGS. 1 to 5 show grain structures in some of these gravity cast arrowbars.

From this initial investigation it can be seen that while T5 treatmentis beneficial to the creep properties of gravity cast ZES3 alloys, it isdetrimental to gravity cast MEZ alloys (Table 3). The creep strengths ofZE53+Zr and both types of MEZ alloy are significantly greater than thatof AE42 alloy, and indeed are considered to be outstanding in the caseof both MEZ alloys in the as-cast (F) condition and the ZES3 withzirconium alloy in the TS condition. The T5 treatment also benefits thetensile properties of ZES3 with zirconium, but has no significant effecton the other three types of alloy (Table 2).

It will also be seen that iron levels have a significant effect oncorrosion rate of all the alloys (Tables 4 and 5). Zinc also has adetrimental effect, and the corrosion resistance of ZE53 was found to bepoor even with low iron content. T5 treatment further reduces thecorrosion resistance of all alloys. In addition, iron levels remaincomparatively high even in the presence of 0.3% Mn (no Zr beingpresent).

When the amount of iron is sufficiently great as to form an insolublephase in the alloy, corrosion is significant. However, when the amountis sufficiently low for all the iron to remain dissolved within thealloy itself, corrosion is far less of a problem, and accordingly MEZalloys contain substantially no iron other than that which may bedissolved in the alloy, and preferably substantially no iron at all.

As a result of further testing, it was found that to obtain a suitablylow iron level, say 0.003%, an addition of at least 6% Zirmax wasnecessary in the case of both MEZ and ZE53. However, if manganese isalso present, the necessary addition of Zirmax (or equivalent amount ofother zirconium provider) is reduced to about 1%.

Casting alloys undergo a certain amount of circulation during thecasting process, and may be expected to undergo an increase in ironcontent by contact with ferrous parts of the casting plant. Iron mayalso be picked up from recycled scrap. It may therefore be desirable toadd sufficient zirconium to the initial alloy to provide a residualzirconium content sufficient to prevent this undesirable increase iniron (up to 0.4 weight percent, preferably no more than 0.2 weightpercent, and most preferably no more than 0.1 weight percent). This maybe found to be more convenient than a possible alternative course ofadding further zirconium prior to recasting.

In one trial, it was found that MEZ material with 0.003% iron resultingfrom a 0.5% Zirmax addition underwent an increase in iron to 0.006% uponremelting, with the zirconium content falling to 0.05%. However, MEZmaterial with 0.001% iron resulting from a 1% Zirmax addition underwentan increase in iron only to 0.002% upon remelting, with the zirconiumcontent remaining substantially constant.

To investigate the properties of HPDC alloys, an ingot of MEZ ofcomposition 0.3% Zn, 2.6% RE (rare earth), 0.003% Fe, 0.22% Mn and 0.06%Zr was cast into test bars using both HPDC and PFHPDC methods. Thedetails of the casting methods are appended (Appendix A).

Analysis of the bars is given in Table 6, where FC1, FC2, FC3respectively represent samples taken at the beginning, middle and end ofthe casting trial. The high Zr figure of the first listed compositionindicates that insoluble zirconium was present, suggesting an error inthe sampling technique.

Table 7 and FIGS. 6 to 8 indicate the measured tensile properties of thetest bars, together with comparative measurements on similar bars ofAE42 alloy. It will be seen that MEZ and AE42 have similar yieldstrengths, but that while AE42 has a superior tensile strength at roomtemperature, the situation is reversed at higher temperatures. Thereappeared to be no useful advantage from the use of the pore freeprocess, either in the bars as cast or after T6 heat treatment.

Table 8 shows the results of corrosion tests on the test bars, andsimilar bars of AE42. It proved difficult to remove all surfacecontamination, and the use of alternative treatments should be noted.Where the cast surface is removed, as in the standard preparation (B),the corrosion rates of MEZ and AE42 appeared similar.

The results of creep measurement on bars of both alloys are shown inTable 9 and in FIG. 9. Despite the scatter of results, it can be seenthat the creep strength of MEZ is far superior to that of AE42.

FIGS. 10 and 11 show the grain structure in a PFHPDC MEZ bars before andafter T6 treatment, and FIG. 12 shows the porosity of an HPDC bar ofMEZ.

As illustrated below, an advantage of the present invention is thatprototypes for an HPDC mass production run can be gravity cast, and, inparticular, can be gravity sand cast, in the same alloy and in the sameconfiguration as required for the HPDC run, while obtaining similartensile properties.

A melt comprising 0.35 weight percent zinc, 2.3 weight percent rareearth, 0.23 weight percent manganese and 0.02 weight percent zirconium(balance magnesium) was manufactured on a 2-tonne scale. A 150 Kg lot ofthe same ingot batch was remelted and cast in the form of an automotiveoil pan configuration both by gravity sand casting and by HPDC.Specimens were cut from three castings in each case, and their tensileproperties measured at ambient temperature, the results being shown inTables 10 and 11 respectively it will be seen that there is a closeresemblance between the tensile properties if the sandcast and diecastproducts.

In a separate test, a further ingot from the same batch was melted, but6 weight percent of Zirmax (33% Zr) was added using conventionalmagnesium foundry practice. The analysis of the resulting melt gave 0.58weight percent zirconium.

A section from a sandcasting made from this melt, of the same automotiveoilpan configuration as above, was tensile tested at ambienttemperature. 0.2% PS was 102 MPa, UTS was 178 MPa, and elongation was7.3%, figures which are very similar to those of Tables 10 and 11.

These results may be contrasted with those for the alloy AE42(Mg-4%Al-2%RE—Mn), not within the present invention, which may be usedfor applications requiring good creep resistance at elevatedtemperatures. In this case, although satisfactory properties can begenerated in HPDC components, as illustrated elsewhere in thisspecification it is impossible to generate satisfactory properties inthe alloy by conventional sand casting techniques.

For example, an alloy AE42 (3.68% Al; 2.0% RE; 0.26 Mn) was cast intosteel chilled “arrow bar” moulds. Tensile properties of specimensmachined from these bars were only 46 MPa (0.2% PS) and 128 MPa (UTS).Similar bars cast in an MEZ alloy gave values as high as 82 MPa (0.2%PS) and 180 MPa (UTS) (0.5% Zn; 2.4% RE; 0.2% Mn).

APPENDIX A TIME OBSERVATION a) MEZ PFHPDC TRIAL 0500 Furnace 1 on,crucible fully charged with half ingot (109 kgs). 1100 Charge fullymolten 650° C. 1315 Melt controlling at 684° C. — surface somewhatdrossy. 0500 Furnace 2 on, remaining melt (approx 20 kg) from pre trialmelted. 1100 Charge fully molten 650° C. 1315 Melt controlling at 690°C. — surface somewhat drossy. Both melts protected with Air + SF₆. Heavyoxide/sulphide skins evident on melt surfaces. 1325 Both halves of diemould preheated with gas torch (fixed half 41° C., moving half 40° C.).Die sleeve preheated with metal ladle poured from Furnace 2. 1330 Diemould further preheated by injection of metal ladle poured from Furnace2. Three injections raised die temperature fixed half to 50° C. andmoving half to 51° C. (FC1 analysis sample ladle poured). 1335 Oxygenswitched on at 100 liters/min. Bar casting begins. Metal supply, ladlepoured from No. 1 furnace for each shot (800 g). Die mould sprayed withgraphite water based inhibited release agent throughout. 1340 Castingstopped after 3 shots metal chilling on ladle. Melt temperature raisedto 700° C. 1343 Re-start casting at 683° C. casting rises to 700° C.Stop casting, adjust stroke of plunger. 1350 Re-start casting. No. 11castings fractured (8 and 10 mm dia bars) both show good fracture. 1400Casting stopped. (14 shots) plunger cleaned of oxide contamination. 1410Restart casting melt temperature 701° C. Fixed half die temperature 71°C. Moving half die temperature 67° C. (FC 2 analysis sample ladlepoured). 1455 Casting complete after 40 shots. 120 tensile bars + 40charpy bars. (FC3 analysis sample ladle poured). NOTE: A further 10PFHPDC shots were carried out following the HPDC trial giving a total of150 tensile bars + 50 charpy bars. Identification of each bar wascarried out by marking each one respectively P-1, P-2, P-3, P-4, etc. b)MEZ HPDC TRIAL 1535 Melt temperature in furnace 1 @ 699° C. Die mouldpreheated with first shot and bars discarded. Fixed half die mouldtemperature 74° C. Moving half die mould temperature 71° C. 1536 Barcasting begins, without oxygen, but with the same casting parameters asthe PFHPDC trial, i.e. Pressure of 800 kgs/cm². 1.2 meters/sec plungerspeed. 100-200 meters/sec at the ingate. Die locking force of 350 tonkg/cm². (FC1 analysis sample ladle poured). 1550 Bars 8 mm dia and 10 mmdia from shots 11 and 12 were fractured. Very slight shrinkage/entrappedair was observed. 1600 Fixed half die mould temperature increases to 94°C. Moving half die mould temperature increased to 89° C. (FC2 analysissample ladle poured after shot 21, temp 702° C.) 1610 Casting stoppeddie mould cooled. Fixed half cooled to 83° C. Moving half cooled to 77°C. 1620 Re-start casting. 1650 Casting complete after 42 shots, 120tensile bars + 42 charpy bars. (FC3 analysis sample ladle poured). NOTE:A further 10 HPDC shots were carried out following this trial giving atotal of 152 tensile bars + 52 charpy bars. Identification of each barwas carried out by marking each one respectively 0-1, 0-2, 0-3, etc. (c)AE42 HPDC Trial 0200 Furnace on, crucible previously fully charged withhalf ingots. 1000 Melt at 680° C. Die heating begins. 1005 Dietemperature at 85° C. 1015 Sleeve heating using melt sample begins. Meltsurface much cleaner than ZC71. Casting surfaces also less discoloured.1240 Casting run begins. 1430 Casting run terminated.

TABLE 1 Melt Zirmax Melt No. Size Kg Alloy Mn Addition % Addition % RE %Zn % Mn % Zr % Fe % DF2218 4.5 ZE53, Zr — 6 3.1 4.9 — 0.67 0.003 DF22194.5 ZE53, Zr — 6 3.0 4.8 — 0.74 0.004 DF2220 4.5 MEZ, Zr — 6 2.9 0.5 —0.52 0.003 DF2221 4.5 MEZ, Zr — 6 3.3 0.6 — 0.49 0.002 DF2222 4.5 ZE53,Mn 0.3 — 3.4 5.0 0.28 — 0.046 DF2223 4.5 ZE53, Mn 0.3 — 3.6 4.9 0.29 —0.051 DF2224 4.5 MEZ, Mn 0.3 — 3.3 0.5 0.28 — 0.039 DF2225 4.5 MEZ, Mn0.3 — 3.3 0.5 0.29 — 0.031

TABLE 2 Tensile Tensile Properties, RT Properties, 177° C. Melt NoCondition YS TS % El YS TS % El DF2218 F 116 176 4.3 83 149 19 DF2219 T5154 203 3.3 111 154 17 DF2220 F 102 173 7.5 65 142 24 DF2221 T5 107 1777.8 66 129 32 DF2222 F 77 134 2.5 63 126 19 DF2223 T5 87 139 2.1 73 12024 DF2224 F 75 141 3.8 55 125 13 DF2225 T5 73 141 2.8 56 112 15 YieldStrength (YS) and Tensile Strength (TS) in MPa % El - PercentageElongation RT - Room Temperature

TABLE 3 Creep Properties of Alloys based on MEZ and ZE53 Compositions at177° C. (Arrow Bars) Time to Initial Initial Reach 0.1% plastic ElasticMelt No. Condition CS (Hrs) Strain (%) Strain (%) DF2218 F 345 0.0080.16 240 DF2219 T5 1128 688 DF2220 F 1050* 0.001 0.13 744 DF2221 T5 124262 DF2222 F 3.5 0.11 0.18 3 DF2223 T5 2.0 0.03 0.15 4.5 DF2224 F 4500*0.10 0.15 1030 DF2225 T5 616 260 *Extrapolated, test terminatedprematurely

Applied stress in all tests, 46 MPa (This is the value, according to Dowdata, required to produce a 0.1% creep strain in 100 hours in HPDC AE42material.) Values in table are individual results.

TABLE 4 Corrosion Fe Content Melt No. Condition Rate (mpy) (%) DF2218 F310 0.004 DF2219 T5 1000 0.004 DF2220 F 18.4 0.003 DF2221 T5 23.2 0.003DF2222 F 450 0.049 DF2223 T5 1150 0.049 DF2224 F 480 0.035 DF2225 T5 4900.035 mpy - mils/year

TABLE 5 Corrosion Rate (mpy) Analysis Bar Nos (Cast) Bar Nos (T5) MeltMn Fe Zr 1 3 5 7 2 4 6 8 DF2290 0.21 0.006 0.05 43 29 59  83 40 42 78130 DF2291 0.14 0.002 0.13 21 17 73 170 20 23 62 960

Each alloy also included 2.5 wt % RE and 0.5 wt % Zn mpy—mils/year;

analysis sample taken before bars were poured

TABLE 6 Die Casting Trial Melt Analysis Casting Analysis (wt %)technique Sample Zn RE Fe Mn Zr Al PFHPDC FC1 0.3 2.3 0.002 0.21 0.11 —FC2 0.3 2.2 0.001 0.21 0.01 — FC3 0.3 2.3 0.001 0.21 0.01 — HPDC FC1 0.32.2 0.001 0.21 0.00 — FC2 0.3 2.3 0.001 0.21 0.02 — FC3 0.3 2.2 0.0010.21 0.01 — AE42 Start 2.2 0.002 0.18 4.1 castings Middle 2.2 0.002 0.194.0 End 2.3 0.002 0.22 4.1 AE42 melt 2.4 0.002 0.26 4.0 (55 ppm Be)

TABLE 7 Specimen Diameter Temp. of Heat 0.2% PS TS % Casting (mm) Test(° C.) Treatment (MPa) (MPa) E1 MEZ 8 20 F 131 198 6 HPDC 100 121 167 11150 107 151 21 177 105 146 33 10 20 138 163 4 100 102 152 12 150 90 14318 177 82 128 22 MEZ 8 20 T6 110 207 8 PFHPDC 100 94 168 22 150 77 14233 177 70 126 37 10 20 F 137 180 6 100 98 168 21 150 88 152 26 150 88152 26 177 86 143 32 MEZ 6.4 20 F 138 175 4 HPDC MEZ 6.4 20 F 145 172 3PFHPDC 6.4 20 T6 133 179 4 AE42 6.4 20 F 128 258 17 HPDC 100 103 199 39150 86 151 46 177 83 127 40

TABLE 8 Corrosion Test Results of HPDC MEZ in Accordance With ASTM B11710 Day Salt Fog Test Original Heat Bar Diam. Corrosion Rate (mpy)Casting Treatment (mm) (A) (B) MEZ F 10 469 74 HPDC 8 109 64 MEZ F 10368 49 PFHPDC 8 195 21 MEZ T6 10 302 41 PFHPDC 8 114 — AE42 F 10 44*PFHPDC mpy - mils/year (A) - Sample preparation involves grit blast withAl₂O₃, pickle in 10% HNO₃ aqueous solution. (B) - Sample preparationinvolves machining away cast surface and polishing sample with abrasivepumice powder.

TABLE 9 Creep properties of HPDC MEZ v AE42 Test Time to 0.1% CreepStrain Temp. Stress (hrs) Casting (° C.) (MPa) 1 2 3 4 5 MEZ 20 120 2272 5 24 PFHPDC 100 100 24 0.8 2 104 150 60 2448 >7000 >4500 177 46 8881392 808 MEZ 20 120 192 36 72 80 HPDC 100 100 568 1128 150 60 2592 46265000* 177 46 832 474 3248 2592 213S AE42 20 120 2 5 PFHPDC 100 100 0.30.3 150 60 12 13 177 46 11 13 *Extrapolated result All testing onspecimens with “as cast” surfaces All specimen dimensions are 8.0 mmdiameter × 32 mm

TABLE 10 Sandcast Tensile Properties Specimen Identity 0.2% PS (MPa) UTS(MPa) E % S1-1 101 131 4 S1-2 102 147 4 S2-1 115 145 4 S2-2 132 147 4S3-1 115 131 8 S3-2 107 147 4 Mean 112 141 4

TABLE 11 (Diecast) Tensile Properties Specimen Identity 0.2% PS (MPa)UTS (MPa) E % D1-1 122 151 4 D1-3 120 1812 10 D2-1 126 199 4 D2-2 104189 6 D2-3 111 167 4 D3-1 122 168 4 D3-2 99 173 6 Mean 115 175 5.5

What is claimed is:
 1. A magnesium base alloy suitable for high pressure die casting consisting of: at least 91.9 weight percent magnesium; 0.1 to 2 weight percent of zinc; 2.1 to 5 weight percent of a rare earth metal component other than yttrium, the ratio of said zinc to said rare earth component being less than 1; less than 0.5 weight percent calcium; 0 to 0.1 weight percent of an oxidation inhibiting element other than calcium; no more man 0.001 weight percent strontium; no more than 0.05 weight percent silver; less than 0.1 weight percent aluminum, at least two elements selected from the group consisting of zirconium, hafniumn, and titanium, the amount of combination greater than 0 and less than 0.4%; incidental impurities of less than about 0.15 weight per cent; and any balance being magnesium.
 2. A magnesium base alloy suitable for high pressure die casting consisting of: at least 91 weight percent magnesium; 0.1 to 2 weight percent of zinc; 2.1 to 5 weight percent of a rare earth metal component other than yttrium, the ratio of said zinc to said rare earth component being less than 1; less than 0.5 weight percent calcium; 0 to 0.1 weight percent of an oxidation inhibiting element other than calcium; greater than 0 and less than 0.4 weight percent of a combination of at least two elements chosen from the group consisting of zirconium, hafnium and titanium; 0 to 0.5 weight percent manganese, provided that at least one of said calcium, oxidation inhibiting element, zirconium, hafniumn, titanium, and manganese is not zero weight percent; no more than 0.001 weight percent strontium; no more than 0.05 weight percent silver; no more than 0.1 weight percent aluminum; incidental impurities of less than 0.15 weight per cent; and any balance being magnesium.
 3. A magnesium base alloy suitable for high pressure die casting consisting of: at least 91.9 weight percent magnesium; 0.1 to 2 weight percent of zinc; 2.1 to 5 weight percent of a rare earth metal component other than yttrium, the ratio of said zinc to said rare earth component being less than 1; less than 0.5 weight percent calcium; 0 to 0.1 weight percent of an oxidation inhibiting element other than calcium; no more than 0.001 weight percent strontium; no more than 0.05 weight percent silver; less than 0.1 weight percent aluminum; at least two elements selected from the group consisting of zirconium hafnium, and titanium, the amount of combination greater than 0 and less than 0.4%; no more than 0.1 weight percent of each of nickel and copper; incidental impurities of less than about 0.15 weight percent; and any balance being magnesium.
 4. An alloy according to claim 1 or 2 which contains no more than 0.05 weight percent aluminum.
 5. A corrosion resistant magnesium-based alloy consisting of: at least 91.9 weight percent magnesium; 0.1 to 2 weight percent of zinc; 2.1 to 5 weight percent of an element having an atomic weight of 57-71 or a mixture of said elements having an atomic weight of 57-71, the ratio of said zinc to said rare earth component being less than 1; greater than 0 and less than 0.4 weight percent of at least two components chosen from the group consisting of zirconium, hafnium, and titanium, at least one optional component chosen from the group consisting of less than 0.5 weight percent calcium, up to 0.1 weight percent of an oxidation inhibiting element other than calcium, and up to 0.5 weight percent manganese, said at least one optional component being chosen such that said alloy contains no more than 0.005 weight percent of incidental undissolved iron; no more than 0.00 1 weight percent strontium; no more than 0.05 weight percent silver, less than 0.05 weight percent aluminum; incidental impurities of less than 0.15 weight per cent; and any balance being magnesium.
 6. An alloy according to claim 2, or 5 wherein there is less than 0.33 weight percent of the elements selected from said group consisting of zirconium, hafnium and titanium.
 7. A cast alloy having the composition according to claim 5 whereby the characteristic creep resistance of said cast alloy is such that the time to reach 0.1 percent creep strain under an applied stress of 46 MPa at 177° C. is greater than 500 hours.
 8. An alloy according to claim 5 wherein said calcium, manganese, oxidation inhibiting element, and zirconium and/or hafnium and/or titanium are chosen such that the cast product of said alloy, after heating to 250° C. for 24 hours has a creep resistance such that the time to reach 0.1 percent creep strain under an applied stress of 46 MPa at 177° C. is greater than 100 hours.
 9. An alloy according to claim 5 wherein said calcium, manganese, oxidation inhibiting element, and zirconium and/or hafnium and/or titaniumn are chosen such that the cast alloy product exhibits a corrosion rate of less than 2.5 mm/year as measured according to the ASTM B 117 Salt Fog Test.
 10. An alloy according to claim 1, 2 or 5 which is substantially free of aluminum.
 11. An alloy according to claim 1, 2 or 5 wherein the rate earth component is cerium, cerium mischmetal or cerium depleted mischmetal.
 12. An alloy according to claim 1, 2 or 5 wherein said rare earth metal component comprises 2.1 to 3 weight percent of said alloy.
 13. An alloy according to claim 1, 2 or 5 wherein said zinc comprises no more than 1 weight percent of said alloy.
 14. An alloy according to claim 13 wherein said zinc comprises no more than 0.6 weight percent of said alloy.
 15. An alloy according to claim 1, 2 or 5 comprising substantially no aluminum and/or substantially no strontium and/or substantially no silver.
 16. A method of producing a cast product wherein high pressure die casting is used in conjunction with an alloy according to claim 1, 2 or
 5. 17. A method according to claim 16 further comprising pore free high pressure die casting.
 18. A cast product produced by the method according to claim
 16. 19. A cast product produced by the method according to claims
 17. 20. A magnesium base alloy consisting of: at least 91.9 weight percent magnesium; 0.1 to 2 weight percent of zinc; 2.1 to 5 weight percent of a rare earth metal component other than yttrium, the ratio of said zinc to said rare earth component being less than 1; less than 0.5 weight percent calcium; 0 to 0.1 weight percent of an oxidation inhibiting element other than calcium; no more than 0.001 weight percent strontium; no more than 0.05 weight percent silver; less than 0.1 weight percent aluminum; at least two elements selected from the group consisting of zirconium, hafnium, and titanium, the amount of combination greater than 0 and less than 0.4%; incidental impurities of less than about 0.15 weight per cent with incidental undissolved iron being present in an amount less than 0.005 weight percent; and any balance being magnesium. 